1. Field of the Invention
The present invention relates to a dielectric ceramic and a laminated ceramic capacitor, and more particularly, relates to a dielectric ceramic suitable for a laminated ceramic capacitor which has small size and high capacitance, and a laminated ceramic capacitor manufactured with the use of the dielectric ceramic.
2. Description of the Related Art
Conventionally, barium titanate based compounds with high dielectric constants have been widely used as ceramic materials for use in laminated ceramic capacitors.
With the developments in electronics technology in recent years, the reduction in size and the increase in capacitance for laminated ceramic capacitors have been progressed rapidly.
The laminated ceramic capacitor has dielectric ceramic layers and internal electrodes which are stacked alternately, and has external electrodes formed on both ends of a ceramic sintered body obtained by a firing treatment. The dielectric ceramic layers are reduced in layer thickness to stack a larger number of layers, thereby allowing for a reduction in size and an increase in capacitance for the laminated ceramic capacitors.
In addition, this type of laminated ceramic capacitor is required to have favorable high temperature load characteristics to improve the reliability, so as not to cause defects even when the laminated ceramic capacitor is exposed to high temperature atmosphere for a long period of time. For this reason, the improvement in reliability has been conventionally made by adding a rare earth element to a dielectric ceramic to provide a solid solution of the rare earth element in the main phase grains.
However, when the solid solubility of the rare earth element in the main phase grains is too much, new problems are caused, such as significantly degraded temperature characteristics of capacitance. Thus, the rare earth element is desirably present as a solid solution partly in the main phase grains.
Dielectric ceramics have been developed which have a core-shell structure in which a rare earth element is present as a solid solution only near the surfaces of main phase grains, or have a mixed crystal structure including a mixture of crystal grains of main phase grains with no rare earth element present therein as a solid solution and crystal grains of main phase grains with a rare earth element present therein as a solid solution.
For example, Japanese Patent Application Laid-Open No. 2002-80276 (claim 1, section [0013]) proposes a dielectric porcelain composition including ceramic grains of core-shell structure, in which the core section is composed of domains of almost pure barium titanate, and the shell section is composed of a solid solution containing barium titanate as a main constituent, and the ratio of the number of grains of the entire ceramic forming the sintered body is controlled to 15% or more in a cross section of the sintered body. The control of the ratio of the number of ceramic grains to the total number of ceramic grains to 15% or more allows a laminated ceramic capacitor with a thickness of about 5 μm before firing and with 10 layers stacked to ensure the reliability with favorable lifetime characteristics and provide favorable temperature characteristics of capacitance.